1. Field of the Invention
The present invention relates to an image display device which displays an identical image both on its own display screen such as a display panel and on an external display such as a cathode ray tube (CRT) at the same time.
2. Description of the Prior Art
Recently, the following image display device has been put into practice. This image display device is incorporated in a portable computer such as a lap-top computer having a built-in display panel which is part of the image display device. The image display device is provided with a means for connecting an external display unit such as CRT thereto. Thus, when the external display unit is connected, the image display device can display an identical image both on its own display panel and on the external display unit at the same time.
FIG. 1 is a block diagram of the above image display device. As shown in FIG. 1, the image display device is essentially constituted from a CPU 1, a display control circuit 2, a panel interface 3, an external interface 5, and the display panel 4. An external display unit 6 such as CRT is connected to the external interface 5.
The display control circuit 2 has a display controller and a video random access memory (V-RAM). This circuit outputs display data to the panel interface 3 and the external interface 5. The display data essentially consists of a digital video signal and a synchronous signal for driving the interfaces 3 and 5, the display panel 4 and the external display unit 6. The display data are generated from image information read out from the V-RAM by the display controller.
The panel interface 3 converts the display data output from the display control circuit 2 to data adapted to the display panel 4 (this data will be referred to as panel display data below). Based on the panel display data, the corresponding image is displayed on the panel display 4. On the other hand, the external interface 5 converts the display data output from the display control circuit 2 to data adapted to the external display unit 6 (this data will be referred to as external display data below). Displaying of an image on the external display unit is performed based on the external display data. In this way, the same image is displayed on the display panel 4 and the external display unit 6 at the same time.
The above image display device having a function of displaying an identical image on its display panel and the external display unit at the same time offers a lot of advantages. Especially, the image display device is useful if used as an auxiliary instrument for lectures, presentation, etc., because it is possible to present pictures by means of the external display unit 6 to a lot of people at the same time. However, unfortunately, this device does not have a function of erasing or halting a picture on a screen of the external display unit 6 when required. Therefore, even when one operates the image display device in order to search for a desired image, for example, images on the display panel 4, which are not necessarily required to be presented to an audience, are also displayed on the external display unit 6. In short, the image display device cannot control display conditions of the external display unit.